Power supply output voltage trimming

ABSTRACT

A power supply trim control signal is produced by integrating differences between monitored and target values of the output voltage of a power supply. Register storage requirements are reduced by producing the target value from a nominal voltage value and one of a plurality of margin offsets selected in accordance with control data. The control data also selects between open and closed loop trim control. Stability is enhanced by changing the target value slowly in response to any change in the control data.

This invention relates to trimming the output voltage of a power supply,such as a DC-DC converter or voltage regulator. DC-DC converters arealso referred to as DC, DC-DC, or switch mode regulators, converters,power supplies, POL (point-of-load) modules, etc.; the term “DC-DCconverter” or “converter” is used herein to include all such terms.Voltage regulators operate in a linear mode to provide a regulatedoutput voltage, and desirably have a low drop-out (LDO) voltage betweeninput and output voltages of the regulator. The term “power supply” isused herein to include both converters and regulators.

BACKGROUND

It is known to provide a regulated output voltage from a power supply,the power supply including a closed loop feedback path to maintain itsregulated output voltage at a desired level, referred to as a set-point.For example, a predetermined fraction of the output voltage of the powersupply may be compared with a reference voltage, and the result of thecomparison may be used to control a PWM (pulse width modulation)function of a DC-DC converter or a control element of a linear voltageregulator.

It is also known to provide a power supply with a trim input, to which acontrol signal can be supplied to trim the output voltage of the powersupply. For example, such trimming can be used to adjust the outputvoltage to a desired value, to compensate for changes of the powersupply characteristics due to temperature changes or aging, or formargining, in which for example the output voltage of the power supplyis raised or lowered from the set-point to test operation of a suppliedelectronic circuit at supply voltages above and below its nominal supplyvoltage. This is referred to as active trim or active DC output controlof the power supply.

Active trim can be carried out in an open loop or a closed loop manner.In open loop active trim, the trim input of the power supply iscontrolled in a desired manner without this control being dependent uponmonitoring of the power supply output voltage. With closed loop activetrim, the trim input of the power supply is controlled in a mannerdependent upon, for example, the monitored output voltage of the powersupply.

Consequently, with closed loop active trim there are two closed loopswhich control the power supply. Stable operation of the power supplyrequires that the closed loop active trim not interfere with the powersupply voltage regulating closed loop; this may be difficult to ensureunder all operating conditions, including for example initial start-upconditions and voltage changes for margining.

It is known for example from Summit Microelectronics, Inc. ApplicationNote 39 entitled “SMM105/205/605/665 ADOC Tutorial and PerformanceSummary” dated Jun. 19, 2003, and from an article by Tom DeLurio andGeorge Hall entitled “How you can manage multiple voltages inportables”, Power Management DesignLine newsletter, Mar. 23, 2005, toprovide closed loop active DC output control of a power supply, usingsignificant filtering and signal conditioning of the power supply outputvoltage to produce a voltage which is compared with a reference voltageto control charging of a capacitor. The control signal for the triminput of the power supply is dependent upon the voltage to which thecapacitor is charged.

To reduce the risks of instability, in such a known arrangement theactive DC output control loop has a response that is much slower thanthe response of the power supply voltage regulating closed loop, and inaddition uses a nonlinear digital control element where adjustments ofthe output are always of the same magnitude. However, this introduces aripple voltage on the power supply output voltage. Although, asdescribed in Summit Microelectronics, Inc. Application Note 37 entitled“SMM665, 605, 205, 105 Supply Voltage Marginers and Active DC OutputControllers Component Selection for Optimum Performance” dated Jun. 18,2003, the magnitude of the ripple voltage can be reduced by increasingthe capacitance of the capacitor, this increases the margin settlingtime.

In such power supply control arrangements, parameters such as thenominal voltage, minimum and maximum voltages for margining, and controlparameters may be stored in an EEPROM or other non-volatile memory andmay be downloaded into shadow registers of a control unit on power-up.The control unit may be provided for controlling a plurality of powersupplies, in which case it requires such shadow registers for each ofthe power supplies that it can control. As the shadow registers requirea significant proportion of the total area of an integrated circuitimplementation of the control unit, it is desirable to reduce as much aspossible the size of (number of bits stored by) the shadow registers,without detracting from the operation or accuracy of the control unit.

Accordingly, it is desirable to provide improvements in power supplyoutput voltage trimming and in a control unit for such trimming.

SUMMARY OF THE INVENTION

According to one aspect of this invention there is provided a method oftrimming an output voltage of a power supply, comprising the steps of:storing values representing a nominal value of the output voltage, aplurality of margin values, and control data in a non-volatile store;and, in a control unit: in dependence upon the control data, selectingone of the plurality of margin values and combining it with the nominalvalue of the output voltage to produce a target value of the outputvoltage; monitoring the output voltage of the power supply; integratingdifferences between monitored values of the output voltage and thetarget value to produce a control signal; and supplying the controlsignal to a trim input of the power supply to trim the output voltage ofthe power supply to reduce said differences.

Preferably said values are digital values, the method including the stepof converting the monitored output voltage to a digital value andconverting a digital value representing the control signal to an analogsignal.

In an embodiment of the invention, the step of integrating differencescomprises forming said differences using an arithmetic logic unit andaccumulating the differences in less significant bit positions of anaccumulator having more significant bit positions from which the controlsignal is derived.

Preferably the step of combining a selected one of the margin valueswith the nominal value of the output voltage to produce a target valueof the output voltage is responsive to a change of the control data tomake any change of the target value gradually at a slow rate.

Conveniently the control unit can be responsive to an indication of openloop control in the control data to produce the control signal from aselected one of the plurality of margin values instead of integratingdifferences between monitored values of the output voltage and thetarget value, and to supply the nominal value of the output voltage asthe target value of the output voltage.

Another aspect of the invention provides a control arrangement fortrimming an output voltage of a power supply, comprising: a non-volatilestore for storing values representing a nominal value of the outputvoltage, a plurality of margin values, and control data; and a controlunit comprising: logic responsive to the control data for combining thenominal value of the output voltage with a selected one of the marginvalues to produce a target value of the output voltage; a register forstoring the target value; a monitoring circuit for monitoring the outputvoltage of the power supply to produce monitored values of the outputvoltage; logic for forming differences between the monitored values ofthe output voltage and the target value; and an integrator forintegrating said differences to produce a control signal for supply to atrim input of the power supply to trim the output voltage of the powersupply to reduce said differences.

The integrator can comprise an accumulator having less significant bitpositions to which said differences are supplied and more significantbit positions from which the control signal is derived. The controlarrangement can include a digital-to-analog converter for producing thecontrol signal from a digital value representing the control signal. Themonitoring circuit can comprise an analog-to-digital converter forproducing the monitored values as digital values.

The logic for combining the nominal value of the output voltage with aselected one of the margin values to produce a target value of theoutput voltage can include a circuit responsive to a change of thecontrol data to make any change of the target value occur gradually at aslow rate.

The control unit can be responsive to an indication of open loop controlin the control data to produce the control signal from a selected one ofthe plurality of margin values instead of integrating said differences,and to supply the nominal value of the output voltage as the targetvalue of the output voltage.

The invention also provides a combination of a control arrangement asrecited above and a power supply having a trim input connected toreceive the control signal from the control arrangement for trimming theoutput voltage of the power supply.

A further aspect of the invention provides a controller for a powersupply having a trim input for trimming an output voltage of the powersupply, the controller comprising: an analog-to-digital converter forproducing a digital value representing an output voltage of the powersupply; a digital-to-analog converter for producing a trim signal, forcoupling to the trim input of the power supply, from a digital valuerepresenting the control signal; storage locations for storinginformation for producing the digital value representing the controlsignal, the storage locations including locations for bits of controldata and bits of a target value register; logic responsive to thecontrol data for selecting one of a plurality of digital valuesrepresenting respective margins for the output voltage of the powersupply and combining it with a digital value representing a nominalvalue of an output voltage of the power supply to produce a digitalvalue representing a target value of the output voltage of the powersupply and for storing this in the target value register; logic forforming a difference between the target value in the target valueregister and the digital value representing the output voltage of thepower supply; and an integrator for integrating successive values ofsaid difference to produce the digital value representing the controlsignal.

The controller can include a non-volatile store for storing the controldata and digital values representing said margins and nominal value ofthe output voltage of the power supply. The invention further extends tothe combination of a controller as recited above and a power supply, thepower supply having an output coupled to an input of theanalog-to-digital converter and having a trim input coupled to receivethe trim signal from the digital-to-analog converter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood from the following descriptionby way of example with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates a DC-DC converter with an active trimcontrol unit;

FIG. 2 illustrates a data downloading arrangement for the control unitof FIG. 1 in accordance with an embodiment of this invention;

FIG. 3 is a functional diagram illustrating closed loop control by thecontrol unit of FIG. 1 in accordance with an embodiment of theinvention;

FIG. 4 is a functional diagram illustrating open loop control by thecontrol unit of FIG. 1 in accordance with an embodiment of theinvention; and

FIG. 5 illustrates a modification of the functional diagram of FIG. 3for facilitating stable trim changes.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 illustrates a known form of DC-DCconverter 10, illustrated within a dashed-line box, which operates toproduce a regulated output voltage Vout from an input voltage Vin.

The converter 10 shown in FIG. 1 comprises switches 12 and 14 (typicallyMOSFETs) controlled by a PWM (pulse width modulation) unit 16 forcoupling an input voltage Vin via an inductor 18 to a capacitor 20 toproduce an output voltage Vout. A fraction of the output voltage Vout isproduced by a potential divider, formed by resistors 22 and 24, andsupplied to an inverting (−) input of a comparator 26, a non-inverting(+) input of which is supplied with a reference voltage Vref. An outputof the comparator 26 controls the PWM unit 16 in a closed loop feedbackcontrol arrangement, whereby the output voltage Vout is regulated inaccordance with the reference voltage Vref and parameters of thecircuit, in particular resistances of the resistors 22 and 24.

The junction of the resistors 22 and 24 also constitutes a trim input 28of the converter 10, which can be used as described below to adjust ortrim the output voltage Vout.

It will be appreciated that the converter 10 as shown in FIG. 1 is givenpurely by way of example of a power supply and not by way of limitation.The power supply can have any desired form, and in particular it can beany switch mode converter or it can be a linear voltage regulator. Theinvention is not limited to any specific form of power supply, and theform of converter 10 shown in FIG. 1 is given only to assist in fullydescribing an embodiment of the invention.

FIG. 1 also shows an active trim control unit coupled to the converter10, including a digital-to-analog converter (DAC) 30 which produces anoutput voltage which is coupled via a resistor 32 to the trim input 28of the converter 10. The active trim control unit also includes acontrol unit 34 for supplying a digital output to the DAC 30, and ananalog-to-digital converter (ADC) 36 having a digital output to thecontrol unit 34. The control unit 34 can receive data from anon-volatile store 38, for example an EEPROM, as described below, andhas a serial interface 40 for communications with a separate device suchas a computer.

As shown in FIG. 1, the output voltage Vout of the converter 10 is alsosupplied to a potential divider, formed by resistors 42 and 44, atapping point of which provides an input voltage to the ADC 36 formonitoring of the converter output voltage Vout by the active trimcontrol unit.

The active trim control unit of FIG. 1 can operate in either an openloop or a closed loop manner. For open loop operation, the digitalcontrol of the DAC 30 by the control unit 34 to supply a trim controlvoltage to the trim input 28 of the converter 10 is not dependent uponany monitoring of the output voltage Vout of the converter 10 via theADC 36. For closed loop control, a second closed loop via the ADC 36 isused to provide feedback to the control unit 34, facilitating moreprecise trimming of the converter output voltage Vout. In either case,the converter output voltage Vout can be adjusted to a desired valuedifferent from a nominal set-point value, to compensate for changes overtime or with temperature, and/or for example for margining as discussedabove.

For controlling the converter 10 using the active trim control unit, oninitial supply of power to the arrangement control data that is storedin the store 38 is transferred to registers, referred to as shadowregisters, in the control unit 34, and subsequently the information inthe shadow registers is used for the control. The information stored inthe store 38, and the information stored in the shadow registers, can bemodified via the serial interface 40. Although only one converter 10 orpower supply is referred to here, it can be appreciated that the samecomponents (with different shadow registers) of the active trim controlunit can be used multiply for control of a plurality of different powersupplies using respective control channels. Further, it can beappreciated that other control functions, for example for enabling theconverter 10 via an enable input (not shown), and for comparing theoutput voltage Vout with under-voltage and over-voltage thresholds, canalso be provided via the control unit 34.

For example, a single active trim control unit can be provided for aplurality of power supplies, comprising DC-DC converters and/or linearvoltage regulators, provided on a circuit board for supplying respectivesupply voltages to circuits on the circuit board. Accordingly, it isdesirable to minimize the storage that is required in the shadowregisters of the control unit 34 for each power supply that may becontrolled, without detracting from the control functions or theiraccuracy, in order to minimize the size and cost of an integratedcircuit used for implementing the control functions. The sameconstraints do not apply for the store 38, which can have a relativelylarge storage capacity and is only required to be written to and readfrom at relatively slow speeds.

For example, for a conventional open loop control for margining, it maybe desired to download from the store 38 to the shadow registers an8-bit value for each of nominal, minimum, and maximum voltage values ofthe output voltage Vout of the converter 10, and 3 associated controlbits (two for selecting one of the three voltage values, and one forswitching the active trim control on or off by tri-stating the output ofthe DAC 30), for a total of 27 bits for this storage.

FIG. 2 illustrates an arrangement for downloading data from the store 38to registers in the control unit 34, in accordance with an embodiment ofthis invention, which enables the incremental number of storage elementsrequired in the control unit 34 to be reduced.

Referring to FIG. 2, as shown on the left-hand side, active trim datastored in the store 38 for controlling the converter 10 comprises 4 bitsof control data Trimctrl[3:0], a 10-bit nominal voltage value Vnom[9:0]which represents a desired output voltage of the converter 10, and three8-bit offsets from the nominal voltage value, referred to asMarginhigh[7:0], Marginnom[7:0], and Marginlow[7:0]. These are digitaldifferences from the nominal voltage value, used for example formargining as discussed further below. In each case the bracketed numbersrefer to the bits of the respective value in conventional manner; forexample Marginlow[7:0] refers to a digital value called Marginlow having8 bits which are numbered from 7 to 0.

Instead of this data all being downloaded from the store 38 intoregisters of the control unit 34 on power-up as discussed above,requiring a relatively large number of bit locations in the registers ofthe control unit 34, the storage requirements of the control unit 34 arereduced by arranging its registers in a different manner, as describedbelow with reference to the right-hand side of FIG. 2.

More particularly, the control unit 34 includes a shadow register forthe 4-bit control data Trimctrl[3:0], which comprises three control bitsas described above (two bits for selecting one of the three marginvalues, and one bit for tri-stating the output of the DAC 30), and onebit for selecting between open loop and closed loop control by theactive trim control unit. In addition, the control unit 34 includes an8-bit register Margin[7:0] in which the data of that one of theMarginhigh, Marginnom, and Marginlow values that is selected by the twocontrol bits can be stored.

The control unit 34 also includes a 10-bit register Vtarget[9:0] inwhich a digital target value of the output voltage of the controlledconverter 10 is stored. This register is required in any event in thecontrol unit 34, for example for use in determining over-voltage (OV)and under-voltage (UV) conditions by calculation of differences betweenthe actual output voltage Vout of the converter 10, monitored by thecontrol unit 34 via the ADC 36, and the desired (target) voltage value.

FIG. 3 illustrates functioning of the control unit 34 for closed loopcontrol by the active trim control unit, and FIG. 4 illustratesfunctioning of the control unit 34 for open loop control by the activetrim control unit. For comparison, the functional layouts in FIGS. 3 and4 are similar and corresponding references are used in the two figures,but in each figure functional paths that are not used are shown bydashed lines.

The control data Trimctrl downloaded from the store 38 to the controlunit 34 is examined to determine whether closed loop or open loopcontrol is desired, and to determine which of the three margin values isto be used. For closed loop active trim control as shown functionally inFIG. 3, if Marginhigh is selected, then this 8-bit offset value and the10-bit value Vnom are supplied to an ALU (arithmetic logic unit)function 50 where they are added to produce a 10-bit target value whichis loaded into the Vtarget register 52. If Marginlow is selected, thenthis 8-bit offset value and the 10-bit value Vnom are supplied to theALU function 50 where the Marginlow offset is subtracted from the valueVnom, the resulting 10-bit target value again being loaded into theVtarget register 52.

Similarly, the Marginnom value can be selected by the control dataTrimctrl if margining is not required; the 2 bits of the control dataTrimctrl for selecting the respective margin value to be used can alsoallow for the Marginnom offset value to be selectively added to orsubtracted from the value Vnom in the ALU function 50, so that trimadjustments above or below the nominal value Vnom can be determined.

It is observed that the ALU function 50 is global, i.e. the control unit34 requires only one ALU for all of its arithmetic functions for all ofthe power supplies that are controlled. In addition, it is observed thatthe over-voltage (OV) and under-voltage (UV) thresholds referred toabove are offsets from the target voltage in the Vtarget register 52, sothat the closed loop control function of FIG. 3 provides the advantagethat during margining the OV and UV thresholds track the target voltage.

During normal closed loop control, i.e. after any initial start-up delayand after the voltage Vout has exceeded any required start-up voltage,the output voltage Vout of the converter 10 is monitored via the ADC 36and a corresponding 10-bit digital sampled value is stored in aVsamp[9:0] register 54. If active trim is enabled by the control dataTrimctrl, then an ALU function 56 produces a 4-bit difference or errorsignal Verr between the contents of the registers 52 and 54, andsupplies this to the least significant bit positions of a 16-bitintegrator 58, constituted for example by an accumulator which cancomprise an up-down counter. The 8 most significant bits [15:8] of theintegrator 58 are supplied as an 8-bit value to the DAC 30, whichproduces a corresponding trim control voltage Vtrim.

In the event of any change in the control data Trimctrl, these steps arerepeated, with the target voltage being re-determined in accordance withthe control data.

The control function of FIG. 3 requires storage for the 4-bit controlword as well as the 16-bit integrator 58, for a total of 20 bits ofstorage compared with the 27-bit total of the conventional arrangementas discussed above, other storage requirements being similar for the twodifferent control arrangements (for example, the Vtarget register 52 isrequired in each case for comparisons for OV and UV thresholds).

If the control data Trimctrl indicates that open loop control is to beused, then as shown in FIG. 4 the margin value selected by the controldata Trimctrl is loaded into the most significant 8 bit positions of theintegrator 58, the lower 8 bit positions of which are not used in thiscase. The DAC 30 produces the trim voltage Vtrim from this margin valuein the integrator 58. Thus the upper 8 bit positions of the integrator58 correspond to the Margin register of FIG. 2.

As also shown in FIG. 4, in this case the nominal value Vnom is alsoloaded via the ALU function 50 into the Vtarget register 52, to be usedfor OV and UV threshold comparisons in conjunction with the ADC 36,register 54, and ALU function 56 in a similar manner to that describedabove with reference to FIG. 3, except that in this case the OV and UVthresholds do not track the margined output voltage Vout. It can be seenfrom FIG. 4 that in this case there is no output from the ALU function56 to the integrator 58, so that there is no closed feedback loop forthe active trim control.

For open loop control as shown in FIG. 4, the Marginhigh, Marginnom, andMarginlow values can be determined in conventional manner. For example,for the converter 10 to produce its nominal value of the output voltageVout, the value Marginnom can be used in the open loop control functionof FIG. 4 and can be chosen to correspond to the reference voltage Vrefof the converter 10 in FIG. 1. As in the case of the closed loop controlof FIG. 3, in the open loop control of FIG. 4 in the event of any changein the control data Trimctrl the relevant margin value and the valueVnom are reloaded as described above. The open loop control of FIG. 4does not require any bit storage locations in addition to those of theclosed loop control of FIG. 3.

The control loops must operate in a stable manner not only when theoutput voltage Vout is being maintained at a desired value, but also forchanges between desired values, for example when a margin is changed,and for start-up when the output voltage Vout changes from an initialvalue (typically zero or near zero) to a desired value. It has beenfound that controlling the output voltage Vout for a substantiallylinear change, at a relatively slow rate compared with the responsespeed of the closed loop within the converter 10, between a currentvalue and a desired value provides a desirable stability. The use of theintegrator 58 in the closed loop control of FIG. 3 also enhancesstability of this arrangement, because this constitutes a linear system.This is distinct from the known arrangement of a capacitor andcomparator, which is a non-linear system (for example as explained byDeLurio and Hall in the reference identified above) and is more prone toinstability.

To facilitate start-up, the control arrangement can be used in the openloop configuration of FIG. 4 until the output voltage Vout, as monitoredvia the ADC 36, exceeds a threshold value, and the integrator 58 can beinitialized by loading its 8 most significant bit positions with adesired Marginnom value, corresponding to the reference voltage Vref ofFIG. 1 as described above.

FIG. 5 illustrates a modification of the functional diagram of FIG. 3for facilitating subsequent stable trim changes. This can be used witheither the closed loop control of FIG. 3 or the open loop control ofFIG. 4.

Referring to FIG. 5, the 10-bit Vtarget register 52 as described aboveis supplemented by a 10-bit up-down counter 60 and an ALU function 64.Instead of using the output of the register 52 as the target voltagevalue as described above, the output of the counter 60 is used as thetarget voltage value. The counter 60 can be initialized for start-up ina similar manner to the register 52. Subsequent step changes of thevalue in the register 52 are reflected only relatively slowly by thecounter 60.

To this end, the ALU function 64 determines any difference between theoutputs of the register 52 and the counter 60, and in response to anydifference enables the counter 60 to count pulses of a clock signal CKin a direction to reduce the difference to zero. The clock signal CK canhave a slow pulse rate, for example of the order of one pulse every 10ms, so that changes of the count of the counter 60, and hence the outputvoltage Vout of the converter 10, take place very gradually.Consequently, instability due to the presence of the two closed controlloops is substantially avoided.

The relatively slow rate of change of the output voltage Vout providedin this manner also ensures that this output voltage is more easily keptwithin the OV and UV thresholds, which with the closed loop control ofFIG. 3 track the target voltage value as described above.

Although as described above the active trim control unit comprisesdigital functions which operate on digital values, with conversions toand from corresponding analog values being provided by the DAC 30 andthe ADC 36, it can be appreciated that any of these digital functionscan alternatively be carried out in an analog manner on analog values.

Although particular embodiments of the invention are described above, itcan be appreciated that modifications, variations, and adaptations maybe made without departing from the scope of the invention as defined inthe claims.

1. A method of trimming an output voltage of a power supply, comprisingthe steps of: storing values representing a nominal value of the outputvoltage, a plurality of margin values, and control data in anon-volatile store; and in a control unit: in dependence upon thecontrol data, selecting one of the plurality of margin values stored inthe non-volatile store, combining it with the nominal value of theoutput voltage stored in the non-volatile store to produce a targetvalue of the output voltage, and storing the target value in a registerof the control unit; monitoring the output voltage of the power supply;integrating differences between monitored values of the output voltageand the target value to produce a control signal; and supplying thecontrol signal to a trim input of the power supply to trim the outputvoltage of the power supply to reduce said differences.
 2. A method asclaimed in claim 1 wherein said values are digital values, die methodincluding the step of converting the monitored output voltage to adigital value and converting a digital value representing the controlsignal to an analog signal.
 3. A method as claimed in claim 2 whereinthe step of integrating differences comprises forming said differencesusing an arithmetic logic unit and accumulating the differences in lesssignificant bit positions of an accumulator having more significant bitpositions from which the control signal is derived.
 4. A method asclaimed in claim 2 wherein the step of combining a selected one of themargin values with the nominal value of the output voltage to produce atarget value of the output voltage is responsive to a change of thecontrol data to make any change of the target value gradually at a slowrate.
 5. A method as claimed in claim 2 wherein the control unit isresponsive to an indication of open loop control in the control data toproduce the control signal from a selected one of the plurality ofmargin values instead of integrating differences between monitoredvalues of the output voltage and the target value, and to supply thenominal value of the output voltage as the target value of the outputvoltage.
 6. A control arrangement for trimming an output voltage of apower supply, comprising: a non-volatile store for storing valuesrepresenting a nominal value of the output voltage, a plurality ofmargin values, and control data; and a control unit comprising: logicresponsive to the control data for combining the nominal value of theoutput voltage stored in the non-volatile store with a selected one ofthe plurality of margin values stored in the non-volatile store toproduce a target value of the output voltage; a register for storing thetarget value; a monitoring circuit for monitoring the output voltage ofthe power supply to produce monitored values of the output voltage;logic for forming differences between the monitored values of the outputvoltage and the target value; and an integrator for integrating saiddifferences to produce a control signal for supply to a trim input ofthe power supply to trim the output voltage of die power supply toreduce said differences.
 7. A control arrangement as claimed in claim 6wherein the integrator comprises an accumulator having less significantbit positions to which said differences are supplied and moresignificant bit positions from which the control signal is derived.
 8. Acontrol arrangement as claimed in claim 6 and including adigital-to-analog converter for producing the control signal from adigital value representing the control signal.
 9. A control arrangementas claimed in claim 6 wherein the monitoring circuit comprises ananalog-to-digital converter for producing the monitored values asdigital values.
 10. A control arrangement as claimed in claim 6 whereinthe logic for combining the nominal value of the output voltage with aselected one of the margin values to produce a target value of theoutput voltage includes a circuit responsive to a change of the controldata to make any change of the target value occur gradually at a slowrate.
 11. A control arrangement as claimed in claim 6 wherein thecontrol unit is responsive to an indication of open loop control in thecontrol data to produce the control signal from a selected one of theplurality of margin values instead of integrating said differences, andto supply the nominal value of the output voltage as the target value ofthe output voltage.
 12. In combination, a control arrangement as claimedin claim 6 and a power supply having a trim input connected to receivethe control signal from the control arrangement for trimming the outputvoltage of the power supply.
 13. A controller for a power supply havinga trim input for trimming an output voltage of the power supply, thecontroller comprising: an analog-to-digital converter for producing adigital value representing an output voltage of the power supply; adigital-to-analog converter for producing a trim signal, for coupling tothe trim input of the power supply, from a digital value representingthe control signal; storage locations for storing information forproducing the digital value representing the control signal, the storagelocations including locations for bits of control data and bits of atarget value register; logic responsive to the control data forselecting one of a plurality of digital values, stored in a non-volatilestore, representing respective margins for the output voltage of thepower supply and combining it with a digital value, stored in thenon-volatile store, representing a nominal value of an output voltage ofthe power supply to produce a digital value representing a target valueof the output voltage of the power supply and for storing this in thetarget value register; logic for forming a difference between the targetvalue in the target value register and the digital value representingthe output voltage of the power supply; and an integrator forintegrating successive values of said difference to produce the digitalvalue representing the control signal.
 14. A controller as claimed inclaim 13 wherein the integrator comprises an accumulator having lesssignificant bit positions to which the successive values of saiddifference are supplied and more significant bit positions from whichthe digital value representing the control signal is derived.
 15. Acontroller as claimed in claim 13 wherein the logic to produce thedigital value representing the target value of the output voltage of thepower supply includes a circuit responsive to a change of the controldata to make any change of the digital value representing the targetvalue occur gradually at a slow rate.
 16. A controller as claimed inclaim 13 wherein the logic responsive to the control data is responsiveto an indication of open loop control in the control data to producedigital value representing the control signal from a selected one of theplurality of margin values instead of integrating successive values ofsaid difference, and to supply the digital value representing thenominal value of the output voltage of the power supply as the targetvalue.
 17. A controller as claimed in claim 13 and including anon-volatile store for storing the control data and digital valuesrepresenting said margins and nominal value of the output voltage of thepower supply.
 18. In combination, a controller as claimed in claim 13and a power supply, the power supply having an output coupled to aninput of the analog-to-digital converter and having a trim input coupledto receive the trim signal from the digital-to-analog coverter.